There are a number of manufacturing processes involving the formation of products from fluids such as "hot melts" and fluid monomer and polymer feed stocks, for example nylon and polyesters wherein it is essential to operate on a continuous flow basis. Such processes typically embody extrusion, injection, blow molding, coating and spraying techniques, to manufacture a variety of products, such as synthetic textile fibers, plastic tubing, plastic sheets and films, protective coatings, insulation on electrical wire and the like. Because of the difficulties in initially starting up such processes and cleaning the equipment after a shut down, it is highly desirable to operate such processes on a continuous, uninterrupted flow basis.
In most of the processes noted above, it is the usual practice to include a filter unit in the liquid flow stream to effect removal of impurities which, otherwise, could result in the production of defective products, or which could cause clogging of downstream equipment such as spinnerettes or extrusion dies. Naturally, such filters must be cleaned or replaced on a periodic basis. Rather than shutting down the process to clean or replace such filters, it is common practice to utilize a dual filter system to permit continued, uninterrupted operation. That is, the process fluid is routed through one filter system while the other is being cleaned, replaced or on stand-by. When the filter system in use becomes sufficiently fouled that cleaning or replacement is necessary, the process fluid is routed through the other filter system to permit such service. Thus one or the other of the two filter systems is always in use, while the other is not in use, and therefore, available for service or stand-by.
Because of the continuous nature of such processes, it is desirable, if not essential, that the diversion of process fluid from one filter system to the other be accomplished without any significant change in the fluid flow parameters, i.e. without any change in the fluid flow rate, pressure, temperature or the introduction of air into the system, any of which could adversely effect smooth functioning of the downstream equipment and the quality of the product being produced. In view of these demands, the diverter valves for diverting the process fluid from one filter system to the other must be highly specialized valves designed to effect such an uninterrupted change over, and typically include complex systems for bleeding fluid into the unused filter system to purge air therefrom and bring it up to pressure prior to a change over. The valve hardware is further complicated due to the rather rigorous service conditions, in that the process fluids may be highly corrosive, and may be at pressures as high as 5,000 psi and temperatures of 600.degree. F. or more. Also complicating their use, is the fact that two such diverter valves, one for inlet and one for outlet, must be made to operate simultaneously in unison so that the change over can be made without any interruption or change in the flow characteristics.
While a great number of diverter valves have been developed or proposed, with the art continuing to progressively improve such valves, all prior art valves fall short of meeting all the desired requirements. For example, such valves may be unduly large, or difficult to operate at the fluid pressures involved, or difficult to clean, maintain and repair, or have a tendency to freeze up or develop leaks, necessitating frequent repairs.
Another common problem associated with many prior art diverter valves is that the design of the ports through which the hot fluids must pass is such that isolated areas are present which will lead to temporary entrapment and stagnation of a portion of the process fluid. As a result, such stagnated process fluid may be degraded, which will have a deleterious effect on the product being produced. For example, if polyvinyl chloride is being processed, a temporary stagnation thereof may cause the formation of hydrochloric acid and carbon which can contaminate the entire process. As another example, the stagnation of polyethylene will cause it to become cross-linked or carbonized and formed into a gel.
Most prior art diverter valves utilize a rotary plug valve because of its simplicity in construction and operation. Such valves comprise a cylindrical plug fitted within a mating cylindrical cavity in the valve body, with the cylindrical plug containing flow ports which can be aligned with differing flow ports in the valve body by an appropriate rotation of the plug. Such prior art rotary plugs have been shown to have a tendance to freeze or seize up. Additionally, repeated use of the valve will cause wear of the two cylindrical surfaces with the result that the tightness of fit is soon lost so that leakage then results. Because the cylindrical surfaces are fixed, there is no possibility for the cylindrical surfaces of the plug and valve body to be forced together and stop the leak. To eliminate this problem, more advanced diverter valves have utilized a frusto-conical plug so that the plug can be maintained in a tight fitted relationship regardless of wear between the plug and the valve body. Examples of such valves can be found in U.S. Pat. Nos. 3,455,357, Zink and 3,935,108, Forgues. Such a valve construction, however, has an even greater tendency to freeze or seize up. Additionally, should the frusto-conical plug be loosened or otherwise caused to be lifted even slightly from its conical seat in the valve body, leakage of excessive magnitude will result.
In some instances in the prior art, diverter valves have utilized slide plates as the active mechanism to divert the flow, thereby avoiding the more complicated cylindrical or conical valve interfaces, and thereby be able tc maintain closer tolerances at the valve interfaces. Such slide plate valves, however, typically require non-metallic resilient seals to control or prevent leakage. Experience has shown that such seals will not withstand the high pressures and temperatures of service for a significant period of time, and must therefore be frequently replaced.
An improved slide plate diverter valve is disclosed in U.S. Pat. No. 4,334,552, Blanchard, which provides a zero clearance, metal-to-metal contact at the valve interface. The seal at the interface is maintained and controlled by a plurality of bolts which extend along both sides of the slide plate to adjust the interface pressure between the slide plate and the valve body manifold. While this diverter valve is capable of maintaining an excellent seal without the use of resilient materials, and also provides good flow through characteristics to reduce any low flow areas, considerable effort and skill is required to torque all the bolts to the exacting degree essential tc properly tighten the slide plate against the valve body manifold. In addition, the manufacturing tolerances for machining the slide plate and those surfaces in contact therewith are exceptionally close, and accordingly, service and repair thereon is time consuming and costly.